This invention is directed in general to a method for determining the characteristics of a dielectric material and, more particularly, to a non-destructive, yet sensitive, method and apparatus for detecting the presence or absence of voids contained internally of a plastic material.
Specifically, apparatus is provided to create an alternating electric field through the dielectric material and to increase the strength of this field to create a partial electrical discharge. The field is constantly monitored to detect this partial discharge. As used herein, partial electrical discharge refers to the incipient stages of dielectric breakdown characterized by the transfer of small amounts of electrical charge through a gaseous inclusion, or void, within the dielectric. This transfer of charge through the gaseous inclusion modifies the charge distribution within the dielectric, thus instantaneously altering the magnitude of the electric field through the dielectric material. Although other charge transfer quickly compensates for this change in field magnitude, the instantaneous decrease thereof, corresponding to the transfer of charge through a gaseous inclusion, can be detected by several well-known methods.
It is known in the art of dielectric breakdown that partial discharge occurs within a void contained internally of dielectric material at an electric field magnitude significantly below that magnitude required for electrical breakdown of the dielectric. It is further known, that the magnitude of the applied field required to cause partial discharge within a void is dependent upon the size and shape of the void, the molecular composition of the gaseous substance contained in the void (usually air but not necessarily so), the pressure and temperature (i.e. density) of the gaseous substance, and the frequency of the applied field. Where all other factors are known and held constant, the size of the gaseous inclusion can be determined by the magnitude of the applied electric field required to cause partial electrical discharge. See, e.g., Progress in Dielectrics, Vol. 2, Wiley publishers (1959); See also, Plastics for Electrical Insulation, Interscience publishers (1968).
Measurements such as these, to determine the relative size and population of voids within plastic articles without destroying the article, are desirable throughout all areas of plastic manufacture as a relative indication of plastic integrity, quality, uniformity, goodness and strength.
In many plastic applications it is desirable to know the strength of plastic materials to within established limits. Since the strength of plastic material is in large measure determined by its homogeneity, or lack of gaseous voids, a method to directly determine the relative size and population of these voids is obviously desirable.
Further, by including such a process in the manufacture of plastic materials at a point where the plastic material has been produced but has not been formed for a specific application, relative strength of this material can be indirectly determined such that relatively weak materials can be allocated to those applications requiring less strength, and conversely, relatively strong materials can be allocated to those applications requiring higher strength.
Further, in many applications it is desirable for one reason or another to create plastic materials containing voids. Here goodness of plastic material is determined by a uniform population of voids as opposed to a relatively homogenous plastic article. In such applications, a direct measurement of relative population of gaseous inclusions can be made, thereby directly determining the goodness of this article.
As always, there is a need to minimize expense in product testing and to reduce manufacturing costs such that valuable natural resources can be conserved while allowing quality products to be supplied at a lower cost to the consumer.